linux-kernel - Re: [PATCH] memcg: force charge kmem counter too

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Message-ID: <CALvZod6x5iRmcJ6pYKS+jwJd855jnwmVcPK9tnKbuJ9Hfppa-A@mail.gmail.com>
Date:   Mon, 28 May 2018 10:23:07 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: force charge kmem counter too

On Mon, May 28, 2018 at 2:11 AM, Michal Hocko <mhocko@...nel.org> wrote:
> On Sat 26-05-18 15:37:05, Shakeel Butt wrote:
>> On Sat, May 26, 2018 at 11:51 AM, Vladimir Davydov
>> <vdavydov.dev@...il.com> wrote:
>> > On Fri, May 25, 2018 at 11:55:01AM -0700, Shakeel Butt wrote:
>> >> Based on several conditions the kernel can decide to force charge an
>> >> allocation for a memcg i.e. overcharge memcg->memory and memcg->memsw
>> >> counters. Do the same for memcg->kmem counter too. In cgroup-v1, this
>> >> bug can cause a __GFP_NOFAIL kmem allocation fail if an explicit limit
>> >> on kmem counter is set and reached.
>> >
>> > memory.kmem.limit is broken and unlikely to ever be fixed as this knob
>> > was deprecated in cgroup-v2. The fact that hitting the limit doesn't
>> > trigger reclaim can result in unexpected behavior from user's pov, like
>> > getting ENOMEM while listing a directory. Bypassing the limit for NOFAIL
>> > allocations isn't going to fix those problem.
>>
>> I understand that fixing NOFAIL will not fix all other issues but it
>> still is better than current situation. IMHO we should keep fixing
>> kmem bit by bit.
>>
>> One crazy idea is to just break it completely by force charging all the time.
>
> What is the limit good for then? Accounting?
>

Unlike tcpmem, the kmem accounting is enabled by default. No need to
set the limit to enable accounting.

I think my crazy idea was just wrong and without much thought. Though
is there a precedence where the broken feature is not fixed because an
alternative is available?